Method of producing phosphatic fertilizers containing magnesium values



United States Patent 3,291,594 METHOD OF PRODUCING PHOSPHATIC FERTI- LIZERS CONTAINING MAGNESIUM VALUES John D. Nickerson, Lakeland, Fla., assignor to International Minerals & Chemical Corporation, a corporation of New York No Drawing. Filed Oct. 9, 1963, Ser. No. 314,870

Claims. (Cl. 71-41) This invention generally relates to the production of phosphate fertilizers. In a particular aspect it relates to a process for the production of phosphate fertilizers which contain at least fifty-five percent available phosphoric acid. The commercial manufacture of fertilizers from phosphate rock, until recent years, has essentially contemplated only ordinary superpho-sphate and triple superphosphate. Ordinary superphosphate is manufactured by reacting phosphate rock with sulfuric acid. Triple superphosphate, sometimes referred to as treble or double superphosphate is the article of commerce used largely as a fertilizer. It is made by treating ground phosphate rock with phosphoric acid, or with a mixture of phosphoric acid and sulfuric acid. It has a P 0 content of from about 45 to 50% P 0 and is composed predominantly of monocalcium phosphate, Ca(H PO with small amounts of gypsum and fluorine compounds.

In recent years, consumers of fertilizers have urged the production of phosphate fertilizers of higher analysis than conventional triple superphosphate. The Tennessee Valley Authority sought to meet such demands by the development of a highly concentrated, so-called super phosphoric acid, containing about seventy to seventyfive percent phosphorus pentoxide. Acidulation of phosphate rock with super phosphoric acid yields a fertilizer product containing an average of at least about fifty-four percent available phosphoric acid and characterized by acceptable proportions of water-soluble phosphorus pentox-ide and citrate insoluble material. The production of super phosphoric acid is, however, beset by economic and other problems which have induced the fertilizer industry to seek an alternative route to the production of high analysis phosphate fertilizers.

Calcination of triple superphosphate has also been suggested as a method for obtaining a higher P 0 product. While calcination of triple superphosphate does increase the total P 0 content by driving off water, the increase in P 0 is made at the sacrifice of available P 0 content and ammoniation properties and at temperatures necessary to obtain a total P 0 content of at least 54%, the water solubility and ammoniation characteristics are reduced to where the product is generally unsuitable for use in fertilizers.

It is, accordingly, an object of this invention to provide a new method for the production of high analysis phosphate fertilizers.

It is another object of this invention to provide a method for the production of high analysis fertilizers which contain water-soluble magnesium values.

Still another object of this invention is to provide a method for the production of phosphatic fertilizers of high analysis in which the unavailable phosphate content is maintained at a low level.

Still another object of this invention is to provide a method for the manufacture of a phosphatic fertilizer of unusually high analysis which displays remarkably low hydroscopicity on exposure to humid atmospheres.

These and other objects of this invention will become apparent from the following description.

Briefly, the method of this invention comprises preparing a finely comminuted starting material which contains at least 4% by weight of magnesium, calculated as magnesium oxide, the remainder of the feed being calcium carbonate or phosphate rock. The feed is reacted with phosphoric acid in amounts sufficient to provide a reaction product in which the mole ratio of MgO plus CaO to P 0 is about 1. The reaction product is then calcined at a temperature in a range of 240 to 300 C. for a period of at least about 20 minutes to produce a phosphate fertilizer of outstanding properties.

As used in the specification and claims theterm availahle phosphoric acid, commonly abbreviated to APA, is the sum of the water-soluble and the citrate-soluble phosphoric acid. The phosphoric acid content of fertilizers is expressed as Weight percent phosphorus pentoxide (P 0 The term bone phosphate of line, commonly abbreviated to BPL, is also commonly used to express the phosphate content of fertilizers. This is the equivalent of Ca (PO The citrate-soluble phosphoric acid is that part of the total phosphoric acid in a fertilizer that is insoluble in water but soluble in a solution of citrate of ammonia according to the method adopted by the Association of Ofiicial Agricultural Chemists (A.O.A.C.).

The invention generically contemplates triple superphosphate made from dolomite or from any of the various phosphate materials such as phosphate rocks and apatite minerals. Reference is made to the treatise by Waggaman entitled, Phosphoric Acid, Phosphates and Phosphatic Fertilizers, 2nd Ed. 1952, for a disclosure of a representative group of phosphate rocks and apatites and the production of triple superphosphate. Representative phosphate materials include Florida pebble phosphate, Tennessee phosphates, phosphate from the western Jtates of the United States, Senegalese phosphates, Israeli phosphates and the like. Triple superphosphate derived from Florida pebble phosphate is preferred.

While it has been known that the APA content of phosphatic fertilizers can be increased by calcination at a temperature sufficient to remove moisture from the composition, the prior art has avoided calcination at temperatures in excess of C. because treatment at higher temperatures, particularly temperatures above about 240 C., was known to result in a reversion of the available phosphate values to an unavailable, Water-insoluble and citrate insoluble form. It has now been discovered, and this invention is in part based on this discovery, that when magnesium in the form of dolomite, magnesium carbonate, magnesium oxide, or magnesium chloride is admixed with a calcium-containing rock, such as calcium carbonate or phosphate rock, and the mixture acid-ulated with phosphoric acid to produce a phosphatic fertilizer, the product can be calcined to produce a fertilizer of outstanding available phosphoric acid content, having a P 0 content of 55% or greater Without reversion of substantial amounts of the P 0 to an unavailable form.

In accordance with this invention the ac-idulated product is calcined at a temperature in the range of about 240 to 300 C., and preferably at a temperature in the range of about 260 to 300 C. Operation at higher temperatures results in reversion of phosp-hatic values to water-insoluble and citrate-insoluble forms. Calcination at temperatures lower than about 240 C. results in the production of a fertilizer product having a lower available phosphoric acid content than can be produced at higher temperatures. Calcination in the preferred temperature range of 260 to 300 C. additionally results in the production of a fertilizer product which is substantially non-hydroscopic.

Incorporation of at least 4% by weight of the solid mix of magnesium, calculated as magnesium oxide, is essential to the method of this invention. The absence of the stipulated amount of magnesium results in the production, upon calcinati-on, of a product of reduced available P content which contains substantial quantities of unavailable P 0 Further, the incorporation of magnesium Within the stated ranges permits the preparation of a product suitable for subsequent treatment by amrnoniation to produce a satisfactory ammonium phosphate fertilizer. When the product is calcined at a temperature in the range of 260 to 300 C., as is preferred, it it further preferred that the magnesium be present in an amount in the range of 12% by weight to by weight, calculated as magnesium oxide, based on the weight of the solid feed. In this way a product fertilizer of extremely low hydroscopicity is produced.

The magnesium values incorporated in the solid feed in accordance with this invention may be in the form of dolomite, which is a magnesium-calcium carbonate. Dolomite comprises a preferred solid feed for acidulation in accordance with this invention. The process of this invention may also be carried out by mixing with a phosphate rock, such as Florida pebble, a quantity of magnesium carbonate, magnesium oxide, magnesium chloride, or dolomite suflicient to provide a mixture containing at least 4% of magnesium, calculated as magnesium oxide, based on the total weight of the acidulation solids feed. It will be understood that the feed will be comminuted to a highly divided state in accordance with conventional phosphate fertilizer manufacture. Prefer-ably, the feed will be comminuted to a state in which a major portion of the feed is smaller than 100 mesh. The acidulation can be carried out in a conventional manner =usin-g wet process phosphoric acid having a concentration in the range of 30% to 60% by weight P 0 but preferably about 50 to 55% P 0 The acidulation is carried out in a manner conventional to treble 'superphosp'hate manufacture. After a reaction time of a few minutes, during which the reacting materials are mixed, the materials reach a dry, friable state. The reaction product is then subjected to calcination, for example, in a rotary kiln, at a temperature range of 240 to 300 C. for a time of about 20 minutes to three hours.

Example 1 In accordance with a preferred practice of this invention 217 grams of wet process phosphoric acid having 9. P 0 content of 50% by weight was mixed with 6-7.8 grams of dolomite. The dolomite analyzed 16.5% MgO, and 30.9% CaO. The acid and dolomite were mixed at a temperature of 60 C. The mixture set up in solid form after 1 /2 minutes, and reaching a dry, friable state after about 4 minutes. The ratios of dolomite to Wet process acid were calculated to provide a mole ratio of CaO plus MgO to P 0 of 0.96. A number of 24 gram samples of the acidulation product Were placed in a muffle furnace each case, the sample was raised to the stated temperatures over a period of /2 hour and maintained at that temperature (for a period of /2 hour. The chemical and physical properties of the resulting calcined products are set forth in Table I.

TABLE 1 Percent Percent Percent Theoret. Percent by Temp, Total 0.1. of Total NH3, 1b.! wt. Moisture 0 P205 P205 P 05, unit APA pickup,

W8. Rrl

The data of Table I shows that a 60 APA or higher product is obtained at temperatures in excess of 225 C., and that high water solubility is maintained at temperatures up to 275 C. There is no substantial reversion to citrate insolubility even at temperatures far in excess of those contemplated by this invention. Further experiments were performed by calcining at 250 C. and 300 C. an alcidulation product madeusing the same mole ratio of MgO plus CaO to P 0 but employing a dolomite analyzing 18.2% MgO and 32.4% CaO. The results are set forth in Table II.

TAB LE II Temp, 0 Percent Total P205 Percent of Total P205,-

Water Soluble Example 2 In accordance with this invention dolomite was mixed with phosphate rock (Florida pebble) and ground to 200 mesh. The ground feed was acidulatedwith wet process phosphoric acid in amounts suflicient to provide a calculated mole ratio of MgO plus CaO to P 0 of 0.96, and the acidulated mixture was reacted at a temperature of 75 C. The mixes were cured for two days at 65 C. before calcinati-on in a furnace maintained at 240 to 250 C. The analyses of the feed materials were as follows:

TABLE III Rock Analyses 34.8% P205 49.7% (32.0. Acid Analyses 56.3% P 05 1.12% CaO. Dolomite Analyses 18.2% MgO 32.4% 02.0.

The analyses of the uncalcined cured products, and of and treated by calcination at different temperatures. In the calcined products, are set forth in Table IV.

TAB LE IV Cured product Galcined product t 1) 1. b I p i i of r ck y Percent total Percent 0.1. Percent of Percent total Percent 01. Percent of Percent wt. 1 y 3 P 0 P 0 total P205, Percent FA P P205 total P105, I gain at 65% ES. w.s. an

These data indicate the improvement in the quantities of water-soluble P in the products to be expected by dolomite addition to the phosphate rock under the described operating conditions.

claims the invention can 'be practiced otherwise than as specifically described.

5 The embodiments of the invention in which an exclusive Example 3 property or privilege is claimed are defined as follows: The procedure of Example 2 was repeated with the 1. Amethod of making phosphoric fertilizer comprising exception that the mole ratio of magnesium oxide plus preparing a finely comminuted starting material consisting calcium oxide to P 0 was maintained at 0.98, and in essentially of materials of the group consisting of dolomite, Run 3 basic magnesium carbonate was used instead of 10 admixtures of dolomite and phosphate rock, admixtures dolomite, the basic magnesium carbonate being incorpoof magnesium chloride and phosphate rock, and admixrated in an amount equivalent to the magnesium provided tures of magnesium carbonate and phosphate rock, said by the dolomite addition of Run 2. The results are starting material being characterized by magnesium conset forth in Table V. tent (calculated as magnesium oxide) of about 4.0 to

TABLE V Cured product Caleined product g 99 1? Theoret. Percent of TheonNHa, Percent wt. Percent wt. Percent Percent NH 11). Percent Percent Total P 0 lb. NHg/unit Gain 24 hrs., Gain 24 hrs,

Total P205 0.1. P205 Nrga lr nit Total P205 0.1. P205 W.S. APA 37.1% RH 701% RH 0 50.6 2. 7 4.09 61.6 3. 55 59. 4 1. s9- 4, 0 27, 0 30 49. 7 3. 0 4.18 61. 4 4. 72 77. 3 2. 39 2. 5 1 5 BasicMgOOa 49.6 4.5 4. 39 60.6 4.57 86.5 3.17 5.5 29.5

The data show the water solubility improvement which about 20.0% by weight of said material, acidulating said is obtainable using a mixture of dolomite and phosphate 30 starting material with phosphoric acid in an amount suffirock, and that the same results can be obtained when the cient to provide in the acidulation product having a CaO magnesium values are supplied as basic magnesium carplus MgO to P 0 mole ratio of about 1, and calcining bonate. Also, the improvement in ammoniation characthe acidulation product at a temperature in the range of teristics and hydroscopicity levels are noteworthy. 260 to 300 C. for a period of at least 20 minutes to recover a fertilizer containing P 0 values in water-soluble Example 4 form The reactions of Example 2 were again repeated at a 2. The method in accordance With claim 1 in which the MgO plus CaO to P 0 mole ratio of 0.96. In Run 1 only a u t n of said material (calculated as rock and acid were used. In Run. 2 phosphate rock, acid 1168111111 OXlde) 1S 1 the range of 14 t0 and calcium carbonate were used. In Run 3 phosphate 40 The m h d 111 a cord-M66 With claim 1 in which k, id d d l it were d, 1 R 4 h h t said matenal is a mixture of dolomite and phosphate rook. rock, acid and a quantity of magnesium chloride equivalent method n a rdance With claim 2 in which t th magnesium content f th dglomite of R 3 were the time of calcination is within the range of 20 minutes used. The results are set forth in Table VI. In each of t0 3 hOIITS- these runs the calcination was carried out at a temperature 49 The method n accordance With Claim 4 in which of 260 to 265 C. for three hours. id ma erial is dolomite.

TABLE VI References Cited by the Examiner NY t Percent Percent Percent of 50 UNITED STATES PATENTS $331 P205 figf ti 305,249 9/1884 Stillman 71 47 X 1,869,879 8/1932 Balz 71-41 0clfi+acig 7 6 6 59.4 4.80 57,4 9 4/ 1934 MacIntire 71-53 X 1 v a 12 2,021 671 11/1935 Skinner 71 41 R k d 30 d1 t. 60.1 3.95 68.2 Rfifikiifihirrierfeh icf 60.8 3.57 69.1 55 2.137.674 11/1938 Maclntlre 71-41 2,193,842 3/1940 Rex 71-53 2,977,213 3/1961 Maluta et a1. 71-41 The results of this example establish the effectiveness of the magnesium additive Whether incorporated in the form DONALL H SYLVESTER, P i Examiner of magnesium chloride, or in the form of dolomite.

While the invention has been described in detail with ANTHONY SCIAMANNA Examme' respect to the several preferred embodiments thereof, and

T. D. KILEY, Assistant Examiner. 

1. A METHOD OF MAKING PHOSPHORIC FERTILIZER COMPRISING PREPARING A FINELY COMMINUTED STARTING MATERIAL CONSISTING ESSENTIALLY OF MATERIALS OF THE GROUP CONSISTING OF DOLOMITE, ADMIXTURES OF DOLOMITE AND PHOSPHATE ROCK, ADMIXTURES OF MAGNESIUM CHLORIDE AND PHOSPHATE ROCK, AND ADMIXTURES OF MAGNESIUM CARBONATE AND PHOSPHATE ROCK, SAID STARTING MATERIAL BEING CHARACTERIZED BY MAGNESIUM CONTENT (CALCULATED AS MAGNESIUM OXIDE) OF ABOUT 4.0 TO ABOUT 20.0% BY WEIGHT OF SAID MATERIAL, ACIDULATING SAID STARTING MATERIAL WITH PHOSPHORIC ACID IN AN AMOUNT SUFFICIENT TO PROVIDE IN THE ACIDULATION PRODUCT HAVING A CAO PLUS MGO TO P2O5 MOLE RATIO OF ABOUT 1, AND CALCINING THE ACIDULATION PRODUCT AT A TEMPERATURE IN THE RANGE OF 260 TO 300*C. FOR A PERIOD OF AT LEAST 20 MINUTES TO RECOVER A FERTILIZER CONTAINING P2O5 VALUES IN WATER-SOLUBLE FORM. 